Investigating Double Hearing Protection using Human Subjects

With the growing need to protect the hearing of military personnel, the use of double protection, earplugs and earmuffs used in combination, has become more and more prevalent. Furthermore Active Noise Reduction ANR headsets and earplugs appear to offer a means of increasing the attenuation of these double protection systems. However, it has been shown that the attenuation afforded by double protection is not a simple additive process the combined attenuation tends to be less than the sum of the individual earplug and earmuff attenuation figures. Also, at the levels of attenuation provided by double protection, bone conduction pathways start to play an important part in the sound level received at the ear. Most previous assessments of the attenuation afforded by double protection and bone conduction limits have been carried out using the REAT Real Ear At Threshold test technique. However, this technique is limited for predicting the attenuation from active systems due to the masking effect of the residual electronic noise. The study presented here employed both REAT and MIRE Microphone In Real Ear test techniques to investigate the attenuation given by both passive and active noise protection devices when used in combination. The MIRE experimentation was carried out in noise fields up to 120 dBA. The sound pressure level, in the ear shells and ear canals, was monitored via miniature microphones and probe microphones fitted in personally molded earplugs, respectively. The study included testing on 10 subjects, in which all the hearing protectors were fitted as they would be used in the field. From direct measurements of the Sound Pressure Level under the earmuffs during the MIRE procedure, it can be concluded that the attenuation of the earmuff remains constant, implying that there is an interaction between earmuff and earplug that is leading to the degradation of the attenuation afforded by the double protection system.